1. Field of the Invention
The present invention relates to the improved processing of energetic materials, in particular explosives, such as those used in warhead, munitions, and other highly energetic applications. More particularly, the method of this invention allows for the processing of explosives containing high solid loads of 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazatetracyclo[5.5.0.0.5,903,11]-dodecane (also known and referred to herein as xe2x80x9cCL-20xe2x80x9d and xe2x80x9cHNIWxe2x80x9d).
2. Description of Related Art
For most existing weapons systems, one of the most critical ingredients, if not the most critical ingredient, in terms of explosive performance is the energetic filler. CL-20, with its substantial increase in performance output, is an organic oxidizing compound presenting significant opportunities in terms of energy capabilities for explosives. For example, the use of CL-20 as part of the explosive charge in weapons systems may provide, in comparison to conventional energetic fillers, increased antiarmor penetration and enhanced missile/torpedo effectiveness and lethality.
Many cast explosives are manufactured entirely in batch processes. A discussion of conventional batch processes for processing cast energetic materials is set forth in the background section of U.S. Patent No. 5,565,150. Generally, in a batch process, mixing is performed by first introducing liquid ingredients into an appropriate mixer, such as a standard vertical sigma-blade mixer, to form a plasticized binder. Examples of suitable liquid ingredients include polymeric binders, such as hydroxy-terminated polybutadiene, and plasticizers. Among the conventional plasticizers most commonly used with hydroxy-terminated polybutadiene are dioctyladipate (DOA) and isodecyl pelargonate (IDP). After the liquid ingredients have been mixed thoroughly, the solid ingredients are added in a multistep fashion, with an equal proportion typically being added in each step. Because of the high performance expected of modem explosive devices, cast explosive compositions typically contain high solid contents on the order of about 85 wt % to 92 wt %. Examples of solid ingredients include the following: organic energetic fillers such as RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane); inorganic oxidizers, such as ammonium nitrate, ammonium dinitramide, and ammonium perchlorate; and metal fuels, such as aluminum powder. Once the solid ingredients have been blended into the plasticized binder to achieve homogeneity, a curative (e.g., a diisocyanate) and a cure catalyst (e.g., triphenyl bismuth) are added to cure the plasticized binder. Prior to full cure, the mixture is cast, for example, into the case of a warhead or suitable mold.
When an effective amount of plasticizer is used in combination with RDX or HMX as the energetic filler, the solid ingredients are wetted and coated and the mixture takes on a sufficiently low viscosity to create a relatively free-flowing suspension that is castable without too much difficulty. Generally, it is possible to employ an effective amount of plasticizer to establish a free-flowing suspension without causing the plasticizer to exude from the cast explosive.
However, the inventor has found that the conventional plasticizers used for batch mixing do not create a free-flowing suspension when mixed with high solid loads of CL-20. Although CL-20 shares an association with RDX and HMX inasmuch as each of these compounds is categorized as a nitramine, in the context of this conventional processing technique, CL-20 does not share the same synergistic compatibility with conventional plasticizers as RDX or HMX. The addition of high proportions of CL-20 to conventional plasticized binders, such as hydroxy-terminated polybutadiene plasticized in DOA or IDP, produces a high viscosity formulation having poor flowability. As a consequence, expensive processing equipment and high power expenditures are required for stirring large batches of castable explosive compositions containing high CL-20 loads to create the shear forces needed to homogenize the composition. Even with high shear mixing equipment, it is often only possible to attain marginal homogeneity in the cast material. It is also difficult to cast the high viscosity explosive without generating air pockets in the cast explosive. The presence of air pockets in cast explosives is highly undesirable, since air pockets increase impact sensitivity and decrease performance. Attempts to lower the viscosity of the mixture and attain homogeneity in mixing by the addition of high proportions of plasticizer have also been unsuccessful. The amount of plasticizer needed to homogeneously disperse CL-20 loads in conventional plasticized binders and create a free-flowing casting mixture is typically so great that the plasticizer exudes from the cast explosive material. The exuding of plasticizer from the cast explosive material can cause the material to shrink and separate from its surrounding case.
It is, therefore, an object of this invention to fulfill a long-standing need in the art by providing a technique in which an explosive composition characterized by a high solid content, including a high proportion of CL-20 as one of the solid ingredients, can be processed without encountering the above-discussed problems of high viscosity, poor castability, and plasticizer exudation.
In accordance with the principles of this invention, the above and other objects are attained by a method of processing explosives in which at least one plasticizer selected from the group consisting of naphthenic oil and paraffinic oil is mixed with at least one binder that is miscible with the naphthenic/paraffinic oil. The plasticized binder is then combined with solid energetic ingredients, in particular CL-20, and optionally other solid ingredients, such as other energetic fillers, metal fuels, oxidizers, and fillers. The solid ingredients content of the explosive composition is preferably high, meaning that the total amount of polymeric binder and processing oil remaining in the cast explosive material is not more than about 15% by weight. Preferably, the solid ingredients content of the explosive material is in the range of about 85 wt % to about 92 wt %, more preferably 86 wt % to 91 wt %. As referred to herein, solid ingredients means ingredients that are in a solid state at room temperature, and exclude the polymeric binder and the plasticizing processing oils.
Unlike conventional plasticizers, when CL-20 is mixed in a binder plasticized with a sufficient amount of processing oils, the resulting explosive composition has a sufficiently low viscosity to allow for homogeneous mixing of the solid ingredients in the plasticized binder and to establish, prior to curing of the composition, a relatively free-flowing suspension that can be cast into a desired configuration without the formation of air pockets.
Other objects, aspects and advantages of the invention will be apparent to those skilled in the art upon reading the specification and appended claims which explain the principles of this invention.
The method of the present invention produces cast explosives without the need to rely on solvents and high-shear mixing equipment. Instead, the introduction of an effective amount of naphthenic oil and/or paraffinic oil as a plasticizer in accordance with the practice of this invention permits explosive compositions containing high solid ingredients contents to be processed at sufficiently low viscosities to establish free-flowing suspensions.
In accordance with this method, a binder system is prepared by introducing at least one liquid binder and at least one plasticizer comprising naphthenic oil and/or paraffinic oil into a mixer and intimately mixing the binder and plasticizer with each other to form a homogeneous miscible solution. As the filler is added, the viscosity of the mixture increases. Preferably, a multimodal distribution of nitramine particles, in particular a bimodal or trimodal distribution, is used to improve mix viscosity. It is also preferable to add the solid ingredients via a multistep technique. The invention is not limited to this preferred technique; rather, the solid ingredients may be added in a continuous fashion or in one shot. After mixing to homogeneity, the composition is cast into a mold or container and allowed to cross-link and solidify into the desired shape. In order to establish curing, a curative and, optionally, a cure catalyst are added during processing. Although the curative and cure catalyst are preferably (but not necessarily) added last, the cure catalyst may be added into the binder system prior to the addition of the solid ingredients.
Examples of suitable mixers for blending the binder system with the solid ingredients under low-shear conditions include a vertical sigma-blade mixer and, in the case of a continuous process, a twin-screw extruder, although these examples are not intended to be exhaustive of the types of apparatuses that may be used to practice the method of this invention. Mixing is generally performed in a temperature range of about room temperature to about 65xc2x0 C., since mixing above about 70xc2x0 C. can cause premature curing of the composition. Preferably, mixing is performed at about 45xc2x0 C. to about 60xc2x0 C. to lower the mix viscosity, and subsequent cure is performed at 45xc2x0 C. to 70xc2x0 C., preferably about 57xc2x0 C.
Typical formulations for cast explosive compositions include at least 85 wt % solid ingredients, although more commonly the concentration of solid ingredients for cast explosive composition is in a range of about 86 wt % to about 91 wt % of solid loads, based on the total weight of the dry explosive. Lower than about 85 wt % solid loads can impede homogeneous distribution of the solid ingredients, especially when large coarse loads are used. Practicing higher than about 91 wt % solid loads can interfere with mix efficiency due to high viscosities that accompany high solid loading.
Although the amount of nitramine in the explosive composition is dependent upon the intended use of the explosive, the nitramine energetic filler is usually present in the highest concentration than any other single ingredient in the explosive composition. Generally, from 30 wt % to 91 wt % of the total weight of the dry explosive composition is comprised of nitramines. The minimum amount of nitramines used in the explosive composition generally depends upon the intended application of the explosive. For example, high detonation pressure explosives may have nitramine concentrations of about 86 wt % to 91 wt % based on the total dry weight of the explosive composition, whereas fragment-accelerating explosives and blast explosives contain oxidizers and metals that reduce the overall concentration of nitramines to as low as about 30 wt %.
Although CL-20 is contemplated as the nitramine of choice for this invention, it is also within the scope of this invention to use CL-20 in combination with other energetic fillers, including other nitramines. Exemplary nitramines suitable for the energetic composition of this invention include TEX (4,10-dinitro-2,6,8,12-tetraoxa-4,10-diazatetracyclo-5.5.0.05,903,11-dodecane), RDX (1,3,5-trinitro-1,3,5-triaza-cyclohexane), and/or HMX (1,3,5,7-tetranitro-1,3,5,7-tetraaza-cyclooctane). Preferably, at least 30% by weight to 91% by weight, more preferably 86% by weight to 91% by weight, of the total dry explosive composition comprises CL-20 as the nitramine, since the processing oil used in this invention is specifically designed for overcoming problems that conventional processes have in homogeneously mixing CL-20 into a curable plasticized binder and casting the mixture.
Although it is possible, and in some cases preferred, for the energetic filler to exclusively consist of either CL-20 alone or CL-20 in combination with TEX, HMX, and/or RDX, other solid ingredients may optionally be used with these nitramines. A nonexhaustive list of suitable optional solid ingredients includes other energetic fillers, oxidizers, metals, and reinforcing particles or fibers. Representative energetic fillers include NTO (3-nitro-1,2,4-triazol-5-one), NQ (nitroguanidine), TATB (1,3,5-triamino-2,4,6-trinitrobenzene), and DADNE (1,1-diamino-2,2-dinitro ethane). Representative oxidizers suitable for nonaqueous processing include AP (ammonium perchlorate), AN (ammonium nitrate), HAN (hydroxylammonium nitrate), ADN (ammonium dinitramide), HNF (hydrazinium nitroformate) or mixtures thereof. Representative reactive metals include aluminum. The metals and oxidizer may be present as a powder, particles, and/or in other forms. Those skilled in the art will appreciate that other known and novel solid ingredients suitable for explosive compositions and not listed above may also be used in the present invention.
The plasticizerxe2x80x94i.e., the naphthenic oil and/or paraffinic oilxe2x80x94and the binder comprise not more than about 15% by weight of the total weight of the dry explosive composition, more preferably 9% by weight to 14% by weight of the total dry weight of the explosive composition. The plasticizer-to-binder weight ratio is preferably about 1:1. The amount of plasticizer may exceed this preferred weight ratio, so long as controlled to not exude the plasticizer from the cast explosive. Likewise, the amount of binder may be increased above the 1:1 weight ratio, so long as the viscosity of the explosive composition is not raised too high. Generally, the Brookfield viscosity of the explosive composition prior to cure is no higher than 30 kilopoise (kp) at processing temperatures.
The terms xe2x80x9cnaphthenic oilxe2x80x9d and xe2x80x9cparaffinic oilxe2x80x9d are often used interchangeably, since these processing oils typically include paraffins, cycloparaffins, and aromatic components. Naphthenic and paraffinic oils are refined products of crude petroleum. Representative naphthenic oils include, by example and not exclusive of the scope of this invention, STAN PLAS oils, whereas examples of paraffinic oils include SUNPAR.
The presence of the processing oil in accordance with the teaching of this invention obviates the need to use a conventional plasticizer for improving processability. However, it is within the scope of this invention to use one or more conventional plasticizers in combination with the processing oil in order to tailor the physical properties of the cast explosive material. Representative inert plasticizers include DOA (dioctyladipate), IDP (isodecylperlargonate), DOP (dioctylphthalate), DOM (dioctylmaleate), DBP (dibutylphthalate), or mixtures thereof. The substitution of one of these conventional plasticizers for either naphthenic processing oil or paraffinic processing oil will generally raise the viscosity of the castable explosive composition, requiring the weight ratio of plasticizer-to-binder to be raised.
Suitable binders include those miscible with the processing oil and suitable for casting. Preferred binders include one or more members selected from the group consisting of hydroxy-functional polydiolefin prepolyrners, such as HTPB (hydroxy-terminated polybutadiene) and hydroxy-terminated polyisoprene; and carboxy-functional polydiolefin prepolymers, such as CTPB (carboxy-terminated polybutadiene) (available from Morton). Also suitable for the invention is poly (butadiene-co-acrylonitrile-co-acrylic acid) (available from American Synthetic Rubber). Polyacrylates and polymethacrylates reacted with small amounts of a comonomer (e.g., acrylic acid) may be practiced. Suitable polyacrylates that are miscible with the processing oil include poly(hexylacrylate) and poly(ethylhexylacrylate) and their copolymers; the methacrylate analogs to hexyl- and 2-ethylhexylacrylate may also be used. Suitable inert polyoxetanes miscible with the processing oil include poly(dimethyloxetane) and poly(diethyloxetane), or mixtures and copolymers thereof. Halogenated binders may also be used, including, for example, VITON(copyright) (vinylidenefluoride-co-tetrafluoroethylene-co-hexafluoropropylene or vinylidenefluoride-co-tetrafluoroethylene-co-perfluoro(methylvinylether)) and KEL-F(copyright) (vinylidenefluoride-co-chlorotrifluoroethylene).
Small amounts of surfactants, such as lecithin, facilitate the adhesion of the binder system to the surfaces of the filler ingredients.
Generally, curatives are added in an amount of from about 0.5 wt % to about 3 wt %, and one or more cure catalysts are added in an amount of from about 0.01 wt % to about 2 wt %. Exemplary curatives for the hydroxy-multifunctional polydiolefin prepolymers and carboxy-functional polydiolefin prepolymers are chain-extending diisocyanates, such as isophoronediisocyanate (IDPI), dimeryldiisocyanate (DDI), toluene diisocyanate (TDI), and tetramethylxylene diisocyanate (TMXDI), although it is within the scope of this invention to use polyisocyanates. The carboxy-functional polydiolefin prepolymers can also be cured with aziridine compounds, for example. Epoxy curatives are suitable for curing the polyacrylates. Exemplary cure catalysts include Lewis acids, triphenylbismuth, and alkyltin compounds, such as dibutyltindiluarate.
Various forms of munitions, ordnances, warheads, and other explosive devices may be used in connection with this invention. Generally, munitions and ordnances comprise an explosive fill, an outer case (optionally capable of fragmentation into shrapnel), optionally a liner (typically metal), a fuse system, and a detonator. Preparation of such explosive devices is within the purview of those skilled in the art and can be practiced with the explosives of this invention without undue experimentation.
Advantages of this process include improved homogeneity in the explosive material, enhanced processability during mixing and casting, and lower production costs. The improved homogeneity is attributable to the lower viscosity of the uncured energetic composition. The lower viscosity permits for more thorough blending, especially when solid ingredient addition is performed in a multistep fashion. It will also be appreciated that the lower viscosity imparts greater flowability to the suspension of CL-20 in the binder system, thereby allowing the suspension to be cast into various configurations. Lower production costs arise by circumventing the need for high-shear mixing equipment.